Fin field effect transistor, semiconductor device and fabricating method thereof

ABSTRACT

A substrate is patterned to form trenches and a semiconductor fin between the trenches. Insulators are formed in the trenches and a dielectric layer is formed to cover the semiconductor fin and the insulators. A dummy gate strip is formed on the dielectric layer. Spacers are formed on sidewalls of the dummy gate strip. The dummy gate strip and the dielectric layer underneath are removed until sidewalls of the spacers, a portion of the semiconductor fin and portions of the insulators are exposed. A second dielectric layer is selectively formed to cover the exposed portion of the semiconductor fin, wherein a thickness of the dielectric layer is smaller than a thickness of the second dielectric layer. A gate is formed between the spacers to cover the second dielectric layer, the sidewalls of the spacers and the exposed portions of the insulators.

BACKGROUND

As the semiconductor devices keeps scaling down in size, three-dimensional multi-gate structures, such as the fin-type field effect transistors (FinFETs), have been developed to replace planar Complementary Metal Oxide Semiconductor (CMOS) devices. A structural feature of the FinFET is the silicon-based fin that extends upright from the surface of the substrate, and the gate wrapping around the conducting channel that is formed by the fin further provides a better electrical control over the channel.

For the gate replacement process of the FinFET with short channel (i.e., channel length smaller than 50 nm), a portion of oxide layer covering the silicon-based fin is needed to be etched-off and replaced by a sequentially deposited high-k dielectric layer. However, high etching amount of oxide layer induces leakage path and extrusion path for metal gate formed sequentially.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flow chart illustrating a method for fabricating a semiconductor device in accordance with some embodiments.

FIGS. 2A-2K are perspective views of a method for fabricating a semiconductor device in accordance with some embodiments.

FIGS. 3-5 are cross-sectional views of a semiconductor device in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be follned between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The embodiments of the present disclosure describe the exemplary fabricating process of a semiconductor device which comprises at least one long channel FinFET and at least one short channel FinFET. The semiconductor device may be formed on bulk silicon substrates in certain embodiments of the present disclosure. Still, the semiconductor device may be formed on a silicon-on-insulator (SOI) substrate or a germanium-on-insulator (GOI) substrate as alternatives. Also, in accordance with the embodiments, the silicon substrate may include other conductive layers or other semiconductor elements, such as transistors, diodes or the like. The embodiments are not limited in this context.

Referring to FIG. 1, illustrated is a flow chart illustrating a method for fabricating a semiconductor device in accordance with some embodiments of the present disclosure. The fabricating method at least includes steps S10, step S12, step S14, step S16, step S18, step S20, step 22, step 24 and step S26. First, in step S10, a substrate is provided, and the substrate is then patterned to form a plurality of trenches and a plurality semiconductor fins between the trenches. In step S12, a plurality of insulators are forming in the trenches. The insulators are shallow trench isolation (STI) structures for insulating semiconductor fins, for example. In step S14, a dielectric layer is formed to cover the semiconductor fins and the insulators. In step S16, at least one first dummy gate strip and at least one second dummy gate strip are formed on the dielectric layer, wherein a lengthwise direction of the first and second dummy gate strips is different from a lengthwise direction of the semiconductor fins, and a width of the first dummy gate strip is smaller than a width of the second dummy gate strip. The first dummy gate strip and the second dummy gate strip are conductive strips, such as poly-silicon strips. In step S18, a pair of first spacers and a pair of second spacers are formed on sidewalls of the first and second dummy gate strips respectively. In step S20, the first dummy gate strip is removed and the dielectric layer underneath is thinned to form a thinned portion between the pair of first spacers. In step S22, the second dummy gate strip is removed. In step S24, a first gate is formed between the pair of first spacers to cover the thinned portion and sidewalls of the first spacers. In step S26, a second gate is formed between the pair of second spacers to cover the dielectric layer and sidewalls of the second spacers.

As illustrated in FIG. 1, removal of the first dummy gate strip is performed before removal of the second dummy gate strip. However, removal sequence of the first dummy gate strip (step S20) and the second dummy gate strip (step S22) is not limited in the present disclosure. For example, the first dummy gate strip and the second dummy gate strip may be removed in a same process. Similarly, formation sequence of the first gate (step S24) and the second gate (step S26) is not limited in the present disclosure. For example, the first gate and the second gate may be formed by a same process.

FIG. 2A is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Step 10 in FIG. 1 and as shown in FIG. 2A, a substrate 200 is provided. In one embodiment, the substrate 200 comprises a crystalline silicon substrate (e.g., wafer). The substrate 200 may comprise various doped regions depending on design requirements (e.g., p-type substrate or n-type substrate). In some embodiments, the doped regions may be doped with p-type and/or n-type dopants. For example, the doped regions may be doped with p-type dopants, such as boron or BF₂; n-type dopants, such as phosphorus or arsenic; and/or combinations thereof. The doped regions may be configured for an n-type FinFET, a p-type FinFET or the combination thereof. In some alternative embodiments, the substrate 200 may be made of some other suitable elemental semiconductor, such as diamond or germanium; a suitable compound semiconductor, such as gallium arsenide, silicon carbide, indium arsenide, or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide.

In one embodiment, a pad layer 202 a and a mask layer 202 b are sequentially formed on the substrate 200. The pad layer 202 a may be a silicon oxide thin film formed, for example, by thermal oxidation process. The pad layer 202 a may act as an adhesion layer between the substrate 200 and mask layer 202 b. The pad layer 202 a may also act as an etch stop layer for etching the mask layer 202 b. In at least one embodiment, the mask layer 202 b is a silicon nitride layer formed, for example, by low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). The mask layer 202 b is used as a hard mask during subsequent photolithography processes. Then, a patterned photoresist layer 204 having a predetermined pattern is formed on the mask layer 202 b.

FIG. 2B is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Step S10 in FIG. 1 and as shown in FIGS. 2A-2B, the mask layer 202 b and the pad layer 202 a which are not covered by the patterned photoresist layer 204 are sequentially etched to form a patterned mask layer 202 b′ and a patterned pad layer 202 a′ so as to expose underlying substrate 200. By using the patterned mask layer 202 b′, the patterned pad layer 202 a′ and the patterned photoresist layer 204 as a mask, portions of the substrate 200 are exposed and etched to form trenches 206 and semiconductor fins 208. After the substrate 200 is patterned, the semiconductor fins 208 are covered by the patterned mask layer 202 b′, the patterned pad layer 202 a′ and the patterned photoresist layer 204. Two adjacent trenches 206 are spaced apart by a spacing S. For example, the spacing S between trenches 206 may be smaller than about 30 nm. In other words, two adjacent trenches 206 are spaced apart by a corresponding semiconductor fin 208.

The height of the semiconductor fins 208 and the depth of the trench 206 range from about 5 nm to about 500 nm. After the trenches 206 and the semiconductor fins 208 are formed, the patterned photoresist layer 204 is then removed. In one embodiment, a cleaning process may be performed to remove a native oxide of the semiconductor substrate 200 a and the semiconductor fins 208. The cleaning process may be performed using diluted hydrofluoric (DHF) acid or other suitable cleaning solutions.

FIG. 2C is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Step S12 in FIG. 1 and as shown in FIGS. 2B-2C, an insulating material 210 are formed over the substrate 200 a to cover the semiconductor fins 208 and fill up the trenches 206. In addition to the semiconductor fins 208, the insulating material 210 further covers the patterned pad layer 202 a′ and the patterned mask layer 202 b′. The insulating material 210 may include silicon oxide, silicon nitride, silicon oxynitride, a spin-on dielectric material, or a low-K dielectric material. The insulating material 210 may be formed by high-density-plasma chemical vapor deposition (HDP-CVD), sub-atmospheric CVD (SACVD) or by spin-on.

FIG. 2D is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Step S12 in FIG. 1 and as shown in FIGS. 2C-2D, a chemical mechanical polish process is, for example, performed to remove a portion of the insulating material 210, the patterned mask layer 202 b′ and the patterned pad layer 202 a′ until the semiconductor fins 208 are exposed. As shown in FIG. 2D, after the insulating material 210 is polished, top surfaces of the polished insulating material 210 is substantially coplanar with top surface T2 of the semiconductor fins.

FIG. 2E is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Step S12 in FIG. 1 and as shown in FIGS. 2D-2E, the polished insulating material 210 filled in the trenches 206 is partially removed by an etching process such that insulators 210 a are formed on the substrate 200 a and each insulator 210 a is located between two adjacent semiconductor fins 208. In one embodiment, the etching process may be a wet etching process with hydrofluoric acid (HF) or a dry etching process. The top surfaces T1 of the insulators 210 a are lower than the top surfaces T2 of the semiconductor fins 208. In other words, the semiconductor fins 208 protrude from the top surfaces T1 of the insulators 210 a and sidewalls SW of the semiconductor fins 208 are thus exposed. The height difference between the top surfaces T2 of the fins 208 and the top surfaces T1 of the insulators 210 a is H, and the height difference H ranges from about 15 nm to about 50 nm.

FIG. 2F is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Step S14 in FIG. 1 and as shown in FIGS. 2E-2F, after the insulators 210 a are formed, a dielectric layer 212 is formed to conformally cover the top surface T1 of the insulators 210 a, the top surfaces T2 of the semiconductor fins 208 and the sidewalls SW of the semiconductor fins 208. In one embodiment, the dielectric layer 212 may include silicon oxide, silicon nitride, silicon oxy-nitride, or high-k dielectrics. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and/or mixtures thereof. In one embodiment, the dielectric layer 212 is a high-k dielectric layer with a thickness in the range of about 5 nm to 50 nm. The dielectric layer 212 may be formed by a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation or UV-ozone oxidation. The dielectric layer 212 is thick enough and has good quality to serve as a gate dielectric layer in a long channel FinFET.

FIG. 2G is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Step S16 in FIG. 1 and as shown in FIGS. 2F-2G, at least one first dummy gate strip 214 a and at least one second dummy gate strip 214 b are formed on the dielectric layer 212, wherein a lengthwise direction D1 of the first and second dummy gate strips 214 a, 214 b is different from a lengthwise direction D2 of the semiconductor fins 208. Along the lengthwise direction D1, a first width W1 of the first dummy gate strip 214 a is smaller than a second width W2 of the second dummy gate strip 214 b. The lengthwise direction D1 of the first and second dummy gate strips 214 a, 214 b are perpendicular to the lengthwise direction D2 of the semiconductor fins 208, for example. The number of the first and second dummy gate strips 214 a, 214 b shown in FIG. 2G is merely for illustration, in some alternative embodiments, two or more first and second dummy gate strips 214 a, 214 b may be formed in accordance with actual design requirements. The first and second dummy gate strips 214 a, 214 b include a silicon-containing material, such as poly-silicon, amorphous silicon or a combination thereof. In one embodiment, the first width W1 of the first dummy gate strip 210 a ranges from 5 nm to 50 nm and the second width W2 of the second dummy gate strip 210 b is greater than 50 nm.

In Step S18 in FIG. 1 and as shown in FIG. 2G, after the first and second dummy gate strips 214 a, 214 b are formed, a pair of first spacers 216 a and a pair of second spacers 216 b are formed on sidewalls of the first and second dummy gate strips 214 a, 214 b respectively. As shown in FIG. 2H, the first spacers 216 a are formed on the dielectric layer 212 and extend along the sidewalls of the first dummy gate strip 214 a while the second spacers 216 b are formed on the dielectric layer 212 and extend along the sidewalls of the second dummy gate strip 214 b. The first and second spacers 216 a, 216 b are formed of dielectric materials, such as silicon nitride or SiCON. The first and second spacers 216 a, 216 b may include a single layer or multilayer structure. Since the pair of first spacers 216 a are spaced apart by the first dummy gate strip 214 a, a first gap G1 between the pair of first spacer 216 a substantially equals to the first width W1 of the first dummy gate strip 214 a. Similarly, a second gap G2 between the pair of second spacer 216 b substantially equals to the second width W2 of the second dummy gate strip 214 b.

FIG. 2H is a perspective view of the semiconductor device at one of various stages of the manufacturing method. As shown in FIG. 2H, interlayer dielectric layers 218 are formed to cover the dielectric layer 212 that are not covered by the gate stacks. Top surfaces of the interlayer dielectric layers 218 are substantially coplanar with top surfaces of the first dummy gate strip 214 a and the second dummy gate strip 214 b. In some embodiments, before the interlayer dielectric layers 218 are formed, some processes (e.g., patterning process of dielectric layer 212, fin recessing process, strained source/drain epitaxial process on the semiconductor fins, silicidation process and so on) may be performed in advance. Details of the aforesaid processes are omitted.

FIG. 2I is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Steps S20, S22 in FIG. 1 and as shown in FIGS. 2H-2I, the first dummy gate strip 214 a and the second dummy gate strip 214 b are removed. In one embodiment, the first dummy gate strip 214 a and the second dummy gate strip 214 b are removed, for example, by an etching process. Through properly selecting of etchant, the first dummy gate strip 214 a and the second dummy gate strip 214 b are removed without damaging the interlayer dielectric layers 218, the dielectric layer 212, the first spacers 216 a and the second spacer 216 b significantly. After the first dummy gate strip 214 a and the second dummy gate strip 214 b are removed, a first cavity C1 between the pair of first spacers 216 a and a second cavity C2 between the pair of second spacers 216 b are formed. In other words, the dielectric layer 212 is partially exposed by the first cavity C1 and the second cavity C2.

FIG. 2J is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Steps S20, S22 in FIG. 1 and as shown in FIG. 2J, after the first cavity C1 and the second cavity C2 are formed, a portion of the dielectric layer 212 exposed by the first cavity C1 is partially removed. In other words, the portion of the dielectric layer 212 exposed by the first cavity C1 is thinned to form a thinned portion 212 a between the pair of the first spacers 126 a. During the formation of the thinned portion 212 a, an additional etch process is performed and a portion of the dielectric layer 212 exposed by the second cavity C2 is well protected from being thinned. In one embodiment, the portion of the dielectric layer 212 exposed by the second cavity C2 may be protected and covered by a photoresist layer from being removed. In some embodiments, the remaining thickness of the thinned portion 212 a ranges from 0.2 nm to 5 nm and the thickness of the dielectric layer that is covered by the pair of first spacers 216 a and the pair of second spacers 216 b ranges from 5 nm to 50 nm. As shown in FIG. 2J, the thinned portion 212 a comprises a recess R under the first cavity C1. The thinned portion 212 a of the dielectric layer 212 is thin enough and has good quality to serve as a gate dielectric layer in a short channel FinFET.

FIG. 2K is a perspective view of the semiconductor device at one of various stages of the manufacturing method. In Steps S24, S26 in FIG. 1 and as shown in FIGS. 2J-2K, after the thinned portion 212 a of the dielectric layer 212 is formed, a first gate 222 a is formed in the first cavity C1 and a second gate 222 b is formed in the second cavity C2. For example, the first gate 222 a and the second gate 222 b may be formed by a same process. The first gate 222 a covers the thinned portion 212 a exposed by the first cavity C1 and the second gate 222 b covers the portion of the dielectric layer 212 exposed by the second cavity C2. As shown in FIG. 2K, the extending direction of the recess R substantially parallels with the lengthwise direction (D1) of the first gate 222 a.

In one embodiment, the width W3 of the first gate 222 a ranges from 5 nm to 50 nm and the width W4 of the second gate 222 b is greater than 50 nm. The portion of the semiconductor fin 208 that is overlapped with and covered by the first gate 222 a serves as a channel of a short channel FinFET; and the portion of the semiconductor fin 208 that is overlapped with and covered by the second gate 222 b serves as a channel of along channel FinFET.

As shown in FIG. 2G and FIG. 2K, the width W3 of the first gate 222 a, the width W1 of the first dummy gate strip 214 a and the first gap G1 between the pair of first spacer 216 a are substantially equal (i.e., W3=W1=G1). The width W4 of the second gate 222 b, the width W2 of the second dummy gate strip 214 b and the second gap G2 between the pair of second spacer 216 b are substantially equal (i.e., W4=W2=G2).

FIG. 3 illustrates a cross-sectional view along a cross-section A-A′ of the semiconductor device in FIG. 2K. Referring to FIG. 2K and FIG. 3, top surfaces of the first gate 222 a and the second gate 222 b are substantially coplanar with top surfaces of the first spacers 216 a, the second spacers 216 b and the interlayer dielectric layers 218. In some embodiments, the gate stack may have other suitable modifications, as shown in FIG. 4 and FIG. 5.

As shown in FIG. 4, in some embodiments, a first cap layer 224 a and a second cap layer 224 b are formed to cover the first gate 222 a and the second gate 222 b. The first cap layer 224 a and the second cap layer 224 b may further cover top surfaces of the first spacers 216 a, the second spacers 216 b and the interlayer dielectric layers 218. As shown in FIG. 5, in some alternative embodiments, the first gate 222 a and the second gate 222 b are partially removed from top surfaces thereof so as to respectively form a first gate recess GR1 between the pair of first spacers 216 a and a second gate recess GR2 between the pair of second spacers 216 b. Then, a first cap layer 224 a and a second cap layer 224 b are formed in the first gate recess GR1 and the second gate recess GR2, respectively. It is note that top surfaces of the first gate recess GR1 and the second gate recess GR2 are substantially coplanar with top surfaces of the first spacers 216 a, the second spacers 216 b and the interlayer dielectric layers 218, for example.

In the short channel FinFET (shown in the right portion of FIG. 2K and FIGS. 3-5), the thinned portion 212 a of the dielectric layer 212 is formed to serve as a gate dielectric layer; in the long channel FinFET (shown in the left portion of FIG. 2K and FIGS. 3-5), the dielectric layer 212 between the semiconductor fin 208 and the second gate 222 b serves as a gate dielectric layer. Since the dielectric layer 212 well protects the semiconductor fin 208, leakage paths and extrusion paths for the first and second gate 222 a, 222 b can be prevented. Accordingly, process window of gate replacement process is enlarged. Therefore, yield and reliability of the semiconductor device are enhanced.

In accordance with some embodiments of the present disclosure, a method for fabricating a FinFET includes at least the following steps. A substrate is patterned to form trenches in the substrate and a semiconductor fin between the trenches. A plurality of insulators are formed in the trenches and a dielectric layer is formed to cover the semiconductor fin and the insulators. A dummy gate strip is formed on the dielectric layer, wherein a lengthwise direction of the dummy gate strip is different from a lengthwise direction of the semiconductor fin. A pair of spacers are formed on sidewalls of the dummy gate strip. The dummy gate strip is removed and the dielectric layer underneath is thinned to form a thinned portion between the pair of spacers. A gate is formed between the pair of spacers to cover the thinned portion and sidewalls of the pair of spacers.

In accordance with alternative embodiments of the present disclosure, a method for fabricating a semiconductor device includes at least the following steps. A substrate is patterned to form a plurality of trenches in the substrate and a plurality of semiconductor fins between the trenches. A plurality of insulators are formed in the trenches. A dielectric layer is formed to cover the semiconductor fins and the insulators. At least one first dummy gate strip and at least one second dummy gate strip are formed on the dielectric layer, wherein a lengthwise direction of the first and second dummy gate strips is different from a lengthwise direction of the semiconductor fins, and a width of the first dummy gate strip is smaller than a width of the second dummy gate strip. A pair of first spacers and a pair of second spacers are formed on sidewalls of the first and second dummy gate strips respectively. The first dummy gate strip is removed and the dielectric layer underneath is thinned to form a thinned portion between the pair of the first spacers. The second dummy gate strip is removed. A first gate is formed between the pair of first spacers to cover the thinned portion and sidewalls of the first spacers. A second gate is formed between the pair of second spacers to cover the dielectric layer and sidewalls of the second spacers.

In accordance with yet alternative embodiments of the present disclosure, a semiconductor device including a substrate, a plurality of insulators, a dielectric layer, a first gate, a pair of first spacers, a second gate and a pair of second spacers is provided. The substrate includes a plurality of trenches and a plurality of semiconductor fins between trenches. The insulators are disposed in the trenches. The dielectric layer covers the semiconductor fins and the insulators and includes a thinned portion. The first gate is disposed on the thinned portion. The pair of first spacers are disposed on sidewalls of the first gate. The second gate is disposed on the dielectric layer. The pair of second spacers are disposed on sidewalls of the second gate, wherein a lengthwise direction of the first and second gates is different from a lengthwise direction of the semiconductor fins, and a width of the first gate is smaller than a width of the second gate.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method for fabricating a fin field effect transistor (FinFET), comprising: patterning a substrate to form a plurality of trenches in the substrate and a semiconductor fin between the trenches; forming a plurality of insulators in the trenches; forming a dielectric layer to cover the semiconductor fin and the insulators; forming a dummy gate strip on the dielectric layer, a lengthwise direction of the dummy gate strip being different from a lengthwise direction of the semiconductor fin; forming a pair of spacers on sidewalls of the dummy gate strip; removing the dummy gate strip and thinning the dielectric layer underneath to form a thinned portion between the pair of spacers; and forming a gate between the pair of spacers to cover the thinned portion and sidewalls of the pair of spacers.
 2. The method of claim 1, wherein the pair of spacers are formed on the dielectric layer and extend along the sidewalls of the dummy gate strip before the dummy gate strip is removed.
 3. The method of claim 1, wherein a thickness of the thinned portion ranges from 0.2 nm to 5 nm and a thickness of the dielectric layer covered by the pair of spacers ranges from 5 nm to 50 nm.
 4. The method of claim 1, wherein the thinned portion comprises a recess and the recess is formed by an etch process.
 5. The method of claim 1, wherein the gate having a width between 5 nm an 50 nm is formed between the pair of spacers to cover the thinned portion and the sidewalls of the pair of spacers.
 6. The method of claim 1 further comprising: forming a cap layer to cover the gate and the pair of spacers.
 7. The method of claim 1 further comprising: partially removing the gate to form a gate recess between the pair of spacers; and forming a cap layer in the gate recess to cover the gate.
 8. A method for fabricating a semiconductor device, comprising: patterning a substrate to form a plurality of trenches in the substrate and a plurality of semiconductor fins between the trenches; forming a plurality of insulators in the trenches; forming a dielectric layer to cover the semiconductor fins and the insulators; forming at least one first dummy gate strip and at least one second dummy gate strip on the dielectric layer, wherein a lengthwise direction of the first and second dummy gate strips is different from a lengthwise direction of the semiconductor fins, and a width of the first dummy gate strip is smaller than a width of the second dummy gate strip; forming a pair of first spacers and a pair of second spacers on sidewalls of the first and second dummy gate strips respectively; removing the first dummy gate strip and thinning the dielectric layer underneath to form a thinned portion between the pair of first spacers; removing the second dummy gate strip; forming a first gate between the pair of first spacers to cover the thinned portion and sidewalls of the first spacers; and forming a second gate between the pair of second spacers to cover the dielectric layer and sidewalls of the second spacers.
 9. The method of claim 8, wherein the first and second dummy gate strips are removed in a same process.
 10. The method of claim 8, wherein the pair of first spacers are formed to cover the dielectric layer and extend along the sidewalls of the first dummy gate strip and the pair of second spacers are formed to cover the dielectric layer and extend along the sidewalls of the second dummy gate strip.
 11. The method of claim 8, wherein a thickness of the thinned portion ranges from 0.2 nm to 5 nm and a thickness of the dielectric layer covered by the pairs of first and second spacers ranges from 5 nm to 50 nm.
 12. The method of claim 8, wherein the thinned portion comprises a recess and the recess is formed by an etch process.
 13. The method of claim 8, wherein the first gate having a width between 5 nm and 50 nm is formed between the pair of first spacers to cover the thinned portion and the second gate having a width greater than 50 nm is formed between the pair of second spacers to cover the dielectric layer.
 14. The method of claim 8 further comprising: forming a first cap layer to cover the first gate and the pair of first spacers; and forming a second cap layer to cover the second gate and the pair of second spacers.
 15. The method of claim 8 further comprising: partially removing the first gate and the second gate to respectively form a first gate recess between the pair of first spacers and a second gate recess between the pair of second spacers; and forming a first cap layer and a second cap layer in the first gate recess and the second gate recess respectively.
 16. A semiconductor device, comprising: a substrate comprising a plurality of trenches and a plurality of semiconductor fins between trenches; a plurality of insulators in the trenches; a dielectric layer covering the semiconductor fins and the insulators, the dielectric layer comprising a thinned portion; a first gate disposed on the thinned portion; a pair of first spacers disposed on sidewalls of the first gate; a second gate disposed on the dielectric layer; and a pair of second spacers disposed on sidewalls of the second gate, wherein a lengthwise direction of the first and second gates is different from a lengthwise direction of the semiconductor fins, and a width of the first gate is smaller than a width of the second gate.
 17. The semiconductor device of claim 16, wherein a thickness of the thinned portion ranges from 0.2 nm to 5 nm and a thickness of the dielectric layer covered by the first spacers, the second spacers and the second gate ranges from 5 nm to 50 nm.
 18. The semiconductor device of claim 16, wherein the width of the first gate ranges from 5 nm to 50 nm and the width of the second gate is greater than 50 nm.
 19. The semiconductor device of claim 16, wherein the thinned portion comprises a recess.
 20. The semiconductor device of claim 19, wherein the recess extends along the lengthwise direction of the first gate. 